1. Field of the Invention
The present invention relates to a method for controlling charging and discharging processes with respect to a battery mounted on an electric vehicle, an electric powered carrier vehicle, or the like.
2. Description of the related art
An automated guided vehicle (AGV) is an electrically driven vehicle which uses a battery as at least a part of a power source thereof. For example, AGVs are used as electric powered carrier vehicles for automatically carrying parts at assembly plants for a variety of products. A battery mounted on an AGV is, for example, a high-voltage battery pack including a plurality of rechargeable cells serially connected together.
An AGV typically travels in a plant from a station as a starting point along a predetermined path and returns to the station so as to charge the battery mounted thereon. This operation is repeatedly performed. Electric power consumed by traveling along the predetermined path is usually as slight as substantially 10% of the battery capacity. The battery is not intended to be fully charged for safety reasons, etc., and therefore the battery is repeatedly charged and discharged such that the battery SOC (state of charge) is in a range from about 50% to about 60%, for example.
In the battery repeatedly charged and discharged so as to have a relatively narrow SOC range as described above, a charging memory effect is known to occur. The charging memory effect refers to a phenomenon such as an increase in voltage caused at the last stage of a charging process by repeatedly charging and discharging a battery in a shallow SOC range. In the case where the charging memory effect is caused in a specific SOC range, when an SOC value of the battery is increased so as to be higher than a value in the specific SOC range, a battery voltage is increased, thereby causing a reduction in charging efficiency, for example.
In a basic relationship between the SOC value of the battery and the battery voltage, the battery voltage has a tendency to increase according to an increase in the SOC value. Both in the range of a low SOC value which is in the proximity of 0% and the range of a high SOC value which is in the proximity of 100%, the battery voltage is significantly increased. The battery voltage has a slight tendency to increase between both ranges according to an increase in the SOC value.
The aforementioned AGV employs a system in which, when an excessive charging voltage is detected, a battery of the AGV is forcedly discharged for safety so as to cause the SOC value to be decreased to a lowest level in a range of the SOC values in which the charging and discharging processes are performed. For example, in the case where the range of the SOC values in which the charging and discharging processes are performed is between about 50% and about 60% and a battery voltage corresponding to about 70% of the SOC is set so as to be a highest voltage for charging the battery, the charging and discharging processes are usually performed such that the battery voltage corresponds to between about 50% of the SOC and about 60% of the SOC. When the charging voltage is increased to the predetermined highest level, the battery is forcedly discharged so as to have the SOC value of 50% which is the lowest level in the range of the SOC values in which the charging and discharging processes are performed.
However, when the charging memory effect occurs in the battery and the SOC value of the battery is increased so as to be higher than the SOC value at which the charging memory effect is caused, the battery voltage is rapidly increased, and therefore the battery voltage is erroneously detected as if the value of the battery voltage is at the highest level for charging the battery, so that the battery is forcedly discharged although the level of the battery voltage is lower than the highest level. As a result, the SOC value is decreased to the lowest level in the range of the SOC values in which the charging and discharging processes are performed. In this manner, the SOC value of the battery is forcedly decreased so as not to sufficiently charge the battery, and therefore repeating this operation might cause the battery of the AGV to be incapable of being sufficiently charged.
For example, in the case where the battery voltage corresponding to about 70% of the SOC is set so as to be the highest voltage for charging the battery, when the charging memory effect causes the battery voltage to be considered by a detector (not shown) as being increased to the highest voltage while the actual battery voltage corresponds to about 60% of the SOC, the battery is forcedly discharged so as to have the SOC value of 50%. As a result, the charging and discharging processes are performed with respect to the battery which is not sufficiently charged. In such a state, when the charging memory effect is caused again, the battery is forcedly discharged so that the charging and discharging processes are performed with respect to the battery which is not sufficiently charged. In this manner, when the battery of the AGV is forcedly discharged at a low SOC value due to the charging memory effect, the battery is not sufficiently charged, which may cause the AGV to be incapable of traveling.
The charging memory effect can be prevented by performing a refresh charging and discharging process such that the battery is forcedly discharged so as to have the SOC value of 0% and is fully charged so as to have the SOC value of 100%. However, such a process is usually required to be repeated for a period of several cycles.
FIG. 4 is a graph showing relationships between the number of cycles in the refresh charging and discharging process and the charging memory effect. In FIG. 4, a relationship between the SOC value and the battery voltage denoted by (a) refers to a case where one cycle of the refresh charging and discharging process is performed. In this case, the charging memory effect is caused when the SOC value is between about 50% and about 60%. Another relationship between the SOC value and the battery voltage denoted by (b) refers to a case where two cycles of the refresh charging and discharging processes are performed. In this case, the charging memory effect is caused when the SOC value is between about 60% and about 80%. Still another relationship between the SOC value and the battery voltage denoted by (c) refers to a case where five cycles of the refresh charging and discharging processes are performed. In this case, the charging memory effect is caused when the SOC value is between about 80% and about 100%.
Still another relationship between the SOC value and the battery voltage denoted by (d) refers to a case where six cycles of the refresh charging and discharging processes are performed. Still another relationship between the SOC value and the battery voltage denoted by (e) refers to cases where seven and eight cycles of the refresh charging and discharging processes are performed. In these cases, the charging memory effect is hardly caused. Therefore, it is necessary to perform the refresh charging and discharging processes six times or more so as to prevent the charging memory effect.
A similar problem is caused to a battery mounted on a hybrid electric vehicle (HEV). The battery for a HEV (hereinafter, referred to as xe2x80x9cHEV batteryxe2x80x9d) stores not only electric power for driving an electric motor but also electric power generated in a regenerative cycle. The HEV battery is charged by a heat engine mounted thereon. Therefore, in order to prevent the HEV battery from being rapidly charged, as in the aforementioned case of the battery of the AGV (hereinafter, referred to as xe2x80x9cAGV batteryxe2x80x9d), the HEV battery is charged and discharged so as to have a prescribed SOC value. Accordingly, there is also a possibility that the charging efficiency of the HEV battery can be reduced when the charging memory effect is caused.
According to one aspect of the present invention, there is provided a method for controlling battery charging and discharging in which a battery is charged and discharged such that a SOC (state of charge) value of the battery is increased/decreased to a predetermined range when the SOC value of the battery is in a predetermined range; and a range of the SOC values in which charging and discharging processes are performed sequentially varies and a range of the SOC values after the charging and discharging processes also sequentially varies.
In one embodiment of the invention, both the range of the SOC values in which charging and discharging processes are performed and the range of the SOC values after the charging and discharging processes vary between 10% and 100%.
In another embodiment of the invention, both the range of the SOC values in which charging and discharging processes are performed and the range of the SOC values after the charging and discharging processes vary in stages from a low SOC value to a high SOC value.
In still another embodiment of the invention, charging and discharging processes are performed such that the SOC value is decreased in stages when the range of the SOC values after the charging and discharging processes is between 40% and 100%.
Thus, the invention described herein makes possible the advantages of providing a method for controlling battery charging and discharging which can prevent charging efficiency of a battery from being reduced due to a charging memory effect.